Science

When I founded a new nonprofit organization 15 years ago, the United States and the world urgently needed practical solutions to our energy and climate challenges. That need has only grown more urgent.

Earlier today, I announced my plans to step aside as the President of the Center for Climate and Energy Solutions (C2ES) once my successor is on board. As I look back, I find we have come a long way. That said, any honest assessment of our progress to date in addressing one of this century’s paramount challenges must conclude that we have much, much further to go.

When our organization, then named the Pew Center for Global Climate Change, first launched in 1998, 63 percent of the world’s electricity generation came from fossil fuels. Incredibly, that number is even higher today – 67 percent. The concentration of carbon dioxide in the atmosphere, the main driver of climate change, is also higher than it was then – in fact, at its highest level in more than 2 million years.

Scientists around the globe have just reaffirmed with greater certainty than ever that human activity is warming the planet and threatening to irreversibly alter our climate. Climate change is no longer a future possibility. It is a here-and-now reality. It’s leading to more frequent and intense heat waves, higher sea levels, and more severe droughts, wildfires, and downpours.

We at C2ES have believed from the start that the most effective, efficient way to reduce greenhouse gas emissions and spur the innovation needed to achieve a low-carbon economy is to put a price on carbon. It’s a path that a growing number of countries, states, and even cities are taking.

The Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5) brings policymakers and the public up to date on the state of climate science. The IPCC report, released in stages, is the most comprehensive assessment of existing climate change research and provides a baseline for understanding and action. The Working Group I Summary for Policymakers released Sept. 27, 2013, states with greater certainty than ever that climate change is happening and that human activity is the principal cause. Among the highlights of the report:

The conclusion that much of the warming over the past 50 years is due to human activities is now “extremely likely,” upgraded from “very likely” in the last report.

Estimates of future sea level rise have been significantly increased due to a better understanding of the movement of ice sheets in a warming climate.

The Arctic Ocean is now projected to be ice-free during the summer by mid-century under a high emissions scenario, instead of the end of century as in previous reports.

Human Activity

Each IPCC report has been progressively stronger in attributing climate change to human activities. The AR5 contains the strongest statement yet, saying it is “extremely likely” (a greater than 95 percent chance) that human activities are “the dominant cause of the observed warming” since the 1950s. The Third Assessment (2001) made a similar statement with approximately 66 percent certainty, while the Fourth Assessment Report (AR4) (2007) found that “most of the observed increase in global average temperatures since the mid-20th century is very likely (greater than 90 percent chance) due to the observed increase in anthropogenic greenhouse gas concentrations.”

Global Changes

The AR4 concluded that “warming of the climate system is unequivocal.” The AR5 goes further, concluding that many observed changes (warming of the atmosphere and ocean, sea level rise and melting ice) are “unprecedented over decades to millennia.”

New atmospheric temperature measurements in the AR5 show an estimated warming of 0.85 degrees Celsius (1.5 degrees Fahrenheit) since 1880 with the fastest rate of warming in the Arctic. The AR4 estimated the average warming across the globe over the past century (1906-2005) was 0.74 C (1.33 F).

Sea Level Rise

The AR5 report has significantly increased projected sea level rise over the next century, due to new research that improves understanding of ice sheet movement and melting. The new projections show an increase of 0.26-0.55 meters (10-22 inches) by 2100 under a low emissions scenario and 0.52-0.98 meters (20-39 inches) under the high emissions scenario. The AR4 did not include some of the effects of ice sheet movement due to warming, and therefore published much lower estimates in the range of 0.18-0.38 meters (7-15 inches) under a low emissions scenario and 0.26-0.59 meters (10-23 inches) under a high emissions scenario for sea level rise by 2100.

Sea and Land Ice

The AR5 projects it is likely (greater than 66 percent chance) that the Arctic Ocean will be ice-free during part of the summer before 2050 under a high emissions scenario. This represents a large shift from the AR4, which estimated that the Arctic Ocean would not be ice-free during the summer until late in the 21st century. The AR5 finds that Arctic sea ice surface extent has decreased by 3.5-4.1 percent per decade (9.4-13.6 percent during summer), which is higher than the AR4 estimate of 2.1-3.3 percent per decade (5-9.8 percent during summer). That amounts to between 0.45 and 0.51 million square kilometers (0.17 to 0.2 million square miles) per decade. The AR5 finds these changes unprecedented in at least the last 1450 years.

The AR5 also states that scientists have “high confidence” (80 percent chance) that glaciers have shrunk worldwide, and that the Greenland and Antarctic Ice Sheets have lost mass over the past two decades. The report notes with “very high confidence” (90 percent chance) that ice loss from Greenland has accelerated during the past two decades. Greenland is now losing about 215 gigatonnes (Gt) per year of ice, while the rest of the world’s glaciers lose about 226 Gt per year. For comparison, 200 Gt weighs the same as around 100 billion cars (about 1 billion cars exist on Earth today).

Surface Warming “Pause”

After a period of rapid warming during the 1990s, global mean surface temperatures have not warmed as rapidly over the past decade. The AR5 notes there are “differences between simulated and observed trends over periods as short as 10-15 years (e.g., 1998-2012)”. It concludes that the recent reduction in surface warming is probably due to a redistribution of heat in the ocean, volcanic eruptions, and the recent minimum in the 11-year solar cycle. Most importantly, the report specifically points out that these trends should not undermine our confidence in the “big picture” of our understanding of climate change: “trends based on short records are very sensitive to the beginning and end dates and do not in general reflect long-term climate trends.”

In addition, there is new research proposing explanations for the recent trends that did not make the deadline to be included in the AR5. One paper suggests that some of this “lost” heat is actually in the deep ocean, while another notes that the warming “pause” is actually explained by the unusual number of La Niña (sea surface cooling events) in the Pacific Ocean. The second paper by Yu Kosaka and Shang-Ping Xie states that the “current hiatus is part of natural climate variability, tied specifically to a La-Niña-like decadal cooling. Although similar decadal hiatus events may occur in the future, the multi-decadal warming trend is very likely to continue.”

Cumulative Carbon Budgets

The AR5 relates different carbon “budgets” – an accumulated amount of carbon emissions over time — to the chances of average warming exceeding 2 degrees above 1861-1880 levels. Governments have set an international goal of limiting average warming to 2 C. For the world to have a 50 percent chance of staying below 2 C of warming by 2100, the AR5 identifies a greenhouse gas emissions budget of 840Gt of carbon. More than half of that (over 531GtC) has already been emitted. At current emission rates (around 10 GtC per year), we will use up our carbon budget in just 30 years.

Future Emission Scenarios

The report describes several alternative scenarios of 21st century greenhouse gas concentrations and global temperatures, each associated with different cumulative carbon budgets. Three scenarios represent potential pathways with less warming under various forms of mitigation policy. The fourth represents more of a business-as-usual case, with emissions in the 21st century three to four times larger than the emissions before the 20th century and the highest level of warming in any scenario.

Global surface temperature increases exceed 1.5 C and keep rising beyond 2100 in all scenarios except the lowest-emission scenario, in which actions are taken to nearly eliminate CO2 emissions in the second half of the 21st century. In the scenarios with higher rates of emissions, warming is likely to exceed 2 C by 2100, and could even exceed 4 C.

With the Intergovernmental Panel on Climate Change (IPCC) poised to release its Fifth Assessment of the science underpinning our understanding of climate change, it’s useful to take a step back and recap some of the “big picture” facts.

What is already clear from the science:

Carbon dioxide and other greenhouse gases act to warm the planet.

Carbon dioxide is accumulating in the atmosphere due to emissions from human activities.

The Earth has been warming during the past century. The amount and speed of the warming is unusual compared to past records.

Humans’ emissions of greenhouse gases are largely responsible for this warming.

If emission rates continue, the warming in the 21st century will be much more significant than the warming in the previous century.

In its periodic assessments, the Intergovernmental Panel on Climate Change has expressed growing certainty that global warming is underway and that human activity is a principal cause. The panel’s language has become progressively stronger over time to reflect its growing certainty.

In 1990, the IPCC said that emissions from human activities were “substantially increasing” greenhouse gas concentrations in the atmosphere, which would lead to warming.

By 2013, the panel had concluded that "It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century."

The chart below traces how the IPCC’s conclusions have strengthened over time.

These increases will enhance the greenhouse effect, resulting on average in an additional warming of the Earth’s surface”

1995

Second Assessment

“Most of these studies have detected a significant change and show that the observed warming trend is unlikely to be entirely natural in origin…

…the balance of evidence suggests that there is a discernible human influence on global climate.

…the average rate of warming [in projections for the 21st century] would probably be greater than any seen in the last 10,000 years, but the actual annual to decadal changes would include considerable natural variability.”

2001

Third Assessment

“There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities.

…the projected rate of warming is much larger than the observed changes during the 20th century and is very likely to be without precedent during at least the last 10,000 years, based on paleoclimate data.”

2007

Fourth Assessment

“Warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global average sea level.

Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations.”

2013

Fifth Assessment

"Warming of the climate system is unequivocal, and since the 1950s, many of the observed changes are unprecedented over decades to millennia. The atmosphere and ocean have warmed, the amounts of snow and ice have diminished, sea level has risen, and the concentrations of greenhouse gases have increased.

…It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century."

In 2009, the Obama Administration convened the Interagency Climate Change Adaptation Task Force, and the President signed Executive Order 13514, directing agencies to improve energy and water efficiency, better manage waste and pollution, and reduce greenhouse gas emissions. In addition, the Order requested that agencies identify vulnerabilities and put together a climate adaptation plan by June 2012. The plans were released in February 2013 and began implementation for FY 2013. These Adaptation Plans are often part of an agency’s broader Sustainability Plan and will be updated each year.

Highlighted Adaptation Plans

Other Federal Agency Adaptation Resources

Other federal agencies have published climate change adaptation plans as directed by Executive Order 13514. These agencies are either smaller or have provided fewer details in their adaptation plans; links to the plans are below:

Mayor Michael Bloomberg’s $20 billion plan to safeguard New York City against a future Hurricane Sandy and other climate risks is the most ambitious effort yet by any U.S. city to prepare for the expected impacts of climate change.

The mayor last week announced “A Stronger, More Resilient New York,” a comprehensive plan to protect communities and critical infrastructure, and proposed significant changes to New York’s building codes for new construction and major renovations that will help buildings withstand severe weather and flooding. Its 250 recommendations include building new infrastructure (like installing armor stone shoreline protection in Coney Island), changing how services are provided (like encouraging redundant internet infrastructure), and establishing standardized citywide communication protocols for use during disruptions.

Hurricane season officially starts June 1 and it looks like a busy one in the Atlantic. The National Oceanic and Atmospheric Administration (NOAA) expects a well above-average hurricane season with 13 to 20 named storms. Seven to 11 of them could develop into hurricanes and three to six of those could be major (defined as category 3 or higher). The average over about the past 30 years is 12 named storms, six hurricanes, and three major hurricanes per season.

Photo courtesy NOAA

Tornadoes and Climate Change

Definition of a Tornado

Tornadoes are formed by a combination of atmospheric instability and wind shear. Instability occurs when warm, moist air is wedged under drier, cooler air aloft. This warm air rises, causing the intense updrafts and downdrafts seen in strong thunderstorms — the incubators of tornadoes. Wind shear refers to changes in wind direction and speed at different elevations in the atmosphere. The combination of instability and wind shear forms the rotating column of air that we associate with a tornado. Tornadoes that form over water are known as waterspouts.

The link between tornadoes and climate change is currently unclear. One problem is the difficulty in identifying long-term trends in tornado records, which only date back to 1950 in the United States. Also, the population in many areas affected by tornadoes has grown, so it’s possible that tornadoes in the early part of the 20th century occurred without anyone seeing them. Improved technology, such as advanced radar, also helps us “see” tornadoes that may not have been detected decades ago.

Another problem lies with the physics associated with tornadoes. Researchers are working to better understand how the building blocks for tornadoes -- atmospheric instability and wind shear -- will respond to global warming. It is likely that a warmer, moister world would allow for more frequent instability. However, it is also likely that a warmer world would lessen chances for wind shear. Recent trends for these quantities in the Midwest during the spring are inconclusive. Climate change also could shift the timing of tornadoes or the regions that are most likely to be hit, with less of an impact on the total number of tornadoes.

Adding to the difficulty, tornadoes are too geographically small to be well simulated by climate models. Models can simulate some of the conditions that contribute to forming severe thunderstorms that often spawn tornadoes. Multiple studies (see here and here) find the conditions that produce the most severe thunderstorms are likely to occur more often in a warmer world, even if the total number of thunderstorms decreases (because of fewer weak storms). However, this work does not conclusively tell us whether tornadoes should follow the same trend as their parent thunderstorms.

Threats posed by tornadoes

The most significant threats from tornadoes are the dangers posed by strong winds and debris that is caught up in those winds. Although individual tornadoes may affect a relatively small area compared to large tropical storms, they can threaten people, homes, and communities.

On average, tornadoes in the United States cause 70 deaths and 1,500 injuries per year. The death toll from tornadoes has dropped rapidly because forecasters have more tools to detect dangerous weather and quickly warn people to take shelter.

However, tornadoes still cause billions of dollars a year in property damage. The costliest year on record for tornado damage was 2011, when seven tornado and severe weather outbreaks each caused more than $1 billion in damages, and the total damage for the year was more than $28 billion.

How to Build Resilience

Communities can bolster their resilience and reduce the impacts from tornadoes by:

Adopting more stringent building codes in tornado-prone areas

Continuing to support new severe weather research and improvements to forecasts for severe weather

Heeding watches and warnings when they are issued, and ensuring that individuals can be reached by emergency alert systems (for example, through text message, television, and radio, or via tornado sirens)